Imaging of the eye is important both in order to understand the process of vision and to correct or repair any defects in the vision system. Imaging of the eye is also inherently difficult in several respects. For example, relatively small aperture of the pupil and low reflectivity of the retina limits the amount of light available for imaging with an external instrument to about 10−5˜10−3 of the input, depending on the wavelength. Highly directional lasers and high sensitivity detectors are used in modern instruments such as scanning laser ophthalmoscope (SLO) or optical coherence tomography (OCT). These conventional fundus cameras provide a macroscopic view of the living retina, but they do not have the transverse resolution needed to reveal retinal features on the spatial scale of single cells (˜2 μm). With typical values of the pupil aperture 3 mm, retinal distance 22 mm, and index of refraction 1.33, the numerical aperture of the eye is less than 0.1, which corresponds to diffraction limited resolution of 3.3 μm at the wavelength 0.6 μm. The pupil can be dilated to 5 mm or more but then imperfections, i.e. aberrations, of the cornea and lens prevent diffraction-limited imaging.
Adaptive optics (AO), originally developed for astronomical telescopes, reduces the effect of atmospheric turbulence by measuring the distortion of the wavefront arriving from a point source (guide star) and using the information to compensate for the distortions in the objects to be imaged. When applied to ocular imaging, the “guide star” is provided by a narrow laser beam focused on a spot of the retina. Most commonly a Shack-Hartmann wavefront sensor is used to measure the wavefront of the reflected light. The wavefront distortion is then compensated for using a wavefront corrector, such as deformable mirror or liquid crystal spatial light modulator. The sensor and corrector typically has a few hundred elements, allowing for adjustment of similar number of coefficients in the Zernike aberration polynomials. Several iterations of sensing, computation, and corrections are carried out in a feedback loop to reach a stable state. AO is incorporated in SLO, OCT, and laser refractive surgery. By AO compensation of the aberration, individual photoreceptor cells are resolved in the cone mosaic of the fovea.
Therefore, ophthalmic imaging is limited by the small aperture of the pupil and the aberration of the eye, it is difficult to resolve individual photoreceptor cells of retina. Current technology of adaptive optics solves this problem by employing a wavefront sensor to measure the aberration and a wavefront corrector to compensate for the aberration. But they limit the resolution and speed of the adaptive imaging system and also drive up its cost.